1. Field of the Invention
The present invention relates to an ultrasonic wave motor which drives a movable body by a vibration member which generates a travelling vibration wave.
2. Related Background Art
An ultrasonic wave motor is usually constructed to drive a movable body by a vibration member which generates a travelling vibration wave.
In such an ultrasonic wave motor, the vibration member and the movable body are press-contacted by a pressing mechanism such as a coiled spring or a dish-shaped spring.
FIG. 18 shows a ring-shaped ultrasonic wave motor having a pressing mechanism comprising a plurality of coiled springs. Numeral 11 denotes a holder.
A support member 13 is fitted in the holder 11 and a stator 15 is supported at a center of the support member 13.
A piezoelectric element 17 is integrally bonded to a bottom surface of the stator 15 and a vibration member 19 is formed by the stator 15 and the piezoelectric element 17.
A rotor 21 is rotatably arranged, with a restriction in a radial direction, on a top of the support member 13 through a bearing 28b.
A slide member 23 is integrally bonded to a bottom surface of the rotor 21 and a movable body 25 is formed by the rotor 21 and the slide member 23.
A rotation transmission member 26 is mounted on the top of the rotor 21 integrally with the rotor 21, and the rotation of the rotor 21 is transmitted externally by a notch 26a.
A pressure control member 29 is screwed to the top of the holder 11, and a plurality of coiled springs 31 which form a pressing mechanism are arranged at a predetermined pitch angle between the pressure control member 29 and a pressure transmission member 27.
A thrust bearing 28a is provided between the pressure transmission member 27 and the rotation transmission member 26.
In such an ultrasonic wave motor, when a predetermined voltage is applied to the piezoelectric element 17, a travelling vibration wave is generated in the stator 15, and the slide member 23 and the rotor 21 are rotated by the travelling vibration wave.
The pressure of the coiled spring 31 can be adjusted by rotating the pressure control member 29 to vertically move it to adjust a gap between the pressure control member 29 and the pressure transmission member 27.
FIG. 19 shows an ultrasonic wave motor having a pressing mechanism comprising single coiled spring. In the present ultrasonic wave motor, an integral coiled spring 33 is arranged between the pressure control member 29 and the pressure transmission member 27.
FIG. 20 shows an ultrasonic wave motor having a pressing mechanism comprising a dish-shaped spring. In the present ultrasonic wave motor, a pair of dish-shaped springs 35 are arranged between the pressure control member 29 and the pressure transmission member 27.
In the ultrasonic wave motor which uses the coiled springs 31 or 33 or the dish-shaped spring 35, it is relatively easy to select a small spring constant, and even if the flexure of the spring changes at an initial pressure adjustment or during the use, a variation of the pressure can be made small and an ultrasonic wave motor having a relatively stable performance is provided.
In the ultrasonic wave motor which used the dish-shaped spring 35, the spring constant or a relation between the flexure and the load is not linear but it is a function of the flexure. Accordingly, the variation of the pressure to the variation of the flexure 6 can be made further small by designing such that the dish-shaped spring 35 is used in an area in which the spring coefficient K(.delta.) is substantially constant.
However, in the prior art ultrasonic wave motor, the variation of the pressure is made relatively small but no consideration is paid to the uniformity of the pressure in the contact plane of the vibration member 19 and the movable body 25.
For example, in the ring-shaped ultrasonic wave motor, it is especially desirable that the pressure acts uniformly circumferentially in the ring-shaped contact plane of the stator and the rotor. If it does not uniformly act, it causes instability of performance such as irregular rotation.
It is also desirable that the pressure is uniform radially, but a significant problem does not arise because of a small width in the radial direction.
The uniformity of the pressure in the contact plane is further discussed below.
As shown in FIG. 19, when the single coiled spring 33 is used, the windings are dense and coarse in the vicinity of the bottom winding of the coiled spring 33, and there is a difference between pressures at a point a and a point b of the coiled spring 33. As a result, circumferential load is not uniform.
As shown in FIG. 18, where a plurality of coiled springs 31 are used, a number of small coiled springs are circumferentially arranged. Thus, due to a variation of load by a difference between solidities of the coiled springs 31, there is a difference between pressures and the circumferential load is not uniform.
As shown in FIG. 20, where the dish-shaped spring 35 is used, the pressure transmission member 27 and the dish-shaped spring 35 make circular line contact at a point c and the pressure transmission member 27 receives a pressure from the contact area. Because of the line contact, there are portions at which the pressure transmission member 27 does not contact the dish-shaped spring 35 if there is a circumferential variation in the height of the dish-shaped spring 35. As a result, a difference between pressures is created and a circumferential load is not uniform.
The prior art ultrasonic wave motor further has the following disadvantages.
In the ultrasonic wave motor which uses the coiled springs 31 or 33, when the spring coefficient is to be reduced to reduce the variation of load, it is necessary to increase the flexure, but when the flexure is increased, the total length of the coiled springs 31 or 33 increases and the size of the ultrasonic wave motor including the pressing mechanism increases. As a result, it is very difficult to accommodate the ultrasonic wave motor in a limited space of a camera or an office automation equipment.
In the ultrasonic wave motor which uses the dish-shaped spring 35, a high machining precision is required for the dish-shaped spring 35.
In the dish-shaped spring 35, as shown in FIG. 21, the relation between the flexure .delta. and the load F is not linear, and if the shape of the dish-shaped spring 35 is changed, the relation between the flexure and the load also changes as shown by a, b and c in FIG. 21.
Accordingly, if the shape is determined such that the relation between the flexure and the load is represented by c of FIG. 21 and the spring is used in a flexure range A, the dish-shaped spring exhibits a very small load variation even if the flexure changes to some extent. However, if the work range deviates, the characteristic is not the initially intended one shown by c in FIG. 21 but the one shown by a, b or d. Thus, there is a high risk that the load significantly changes for the variation of flexure in the range A, and hence a high machining precision is required for the dish-shaped spring.